Secret of Mana contains a plentiful amount of unused sprites and animation frames. There are even some completely unused items and characters.
There exists in the ROM an unused, dummied item that acts like a healing item upon use, but has no apparent effects. It is possible that, judging from the way the object looks, that it was meant to be a sprite to show the Weapon Orbs in your inventory.
Using Game Genie code 7EE18056 will change the Boy into a completely whited-out Sprite. There exists several duplicates of the Boy, Girl, and Sprite in the character data, some with alternate palettes (such as for the fight against the clones in Sage Joch's cave), but this white palette is unique to the Sprite and not used anywhere.
Neko Back and Walking FramesEdit
Using Game Genie code 7EE18099 will change the Boy into Neko. Neko never moves from any place where you find him standing in the game, but with this code, you can see that he has unused walking forwards and backwards sprites. He will blink his eyes as he walks. This is also the only way to see him from behind.
Sage Joch Walking FramesEdit
Use Game Genie code 7EE180A3 to turn the Boy in Sage Joch and you will see that he has unused walking forward sprites. Joch never moves from the spot he is standing on during normal gameplay.
Disregard: Sage Joch's walking animation can be activated in the game by talking to him from behind.
Game Genie code 7EE180B3 will change the Boy into an unused character in a white cloak. This character is right between Fanha and Sheex in the character data, so he was most likely meant to be another servant of Emperor Vandole.
Elinee Without HoodEdit
Use Game Genie code 7EE180BA to turn the Boy into the witch, Elinee. Slide left or right to activate an unused sprite of Elinee without her hood.
Cannon Travel Man Back ViewEdit
Use Game Genie code 7EE180CE to turn the Boy into the Cannon Travel Man. You can hold up to turn him around and see an unused back view sprite. During normal gameplay, the Cannon Travel man is always situated against a wall or table so he cannot turn around.
High Stepper AnimationEdit
Game Genie code 7EE180D3 will turn the Boy into a High Stepper. By moving up or down, the High Stepper will display a complete set of unused animation frames of him being squished down and springing up. His eyes bulge out as his neck resets into place. Since this animation was completed, it is surprising that it was not used.
UPDATE: This animation actually is used, but it is very difficult to see because the characters are usually situated in front of the High-Stepper when it cycles through these frames, causing the animation to be hidden behind them.
Game Genie code 7EE180EE turns the Boy into a Snowman. Holding up or down reveals an unused sprite of the Snowman crumbling apart.
Toppled Midge SnowmanEdit
Game Genie code 7EE180EF turns the Boy into the midge version of the snowman. It, too, has an unused frame of it slightly toppled over.
(Code Source: Luis Contreras)